An understanding of population regulation is central to ecology. Murdoch (1994) observed that ‘Persistent species must consist of regulated single populations or regulated collections of subpopulations. Regulation should therefore be ubiquitous in natural populations, provided we look for it at the appropriate spatial (and temporal!) scale.’ Since most of us, I would imagine, think of extinction as a rare thing, most of us consider that ‘density dependent’ controls must act at some population size. Populations must be repressed by factors which ‘intensify as the population density increases and relax as the density falls’, the definition of Huffaker and Messenger (1964). But density dependence turns out to be more difficult to demonstrate in natural populations than one might expect. Frustration may have driven some ecologists away from the study of population regulation entirely (Krebs 1991, 1992).

The reduced emphasis I may seem to place on density independent factors in the present chapter may surprise many of my colleagues. Molluscs are among the most conspicuously successful colonizers of unstable freshwater habitats, and unquestionably show great volatility in local population size, linked to vagaries of their environment. The densities of the Bulinus globosus and Biomphalaria pfeifferi populations inhabiting small rivers on the Zimbabwean highveld may fluctuate to near zero as a function of temperature and rainfall (Woolhouse and Chandiwana 1990a,b, Woolhouse 1992). But viewed at a larger scale, the densities of B. glabrata and B. pfeifferi in Zimbabwe are much more constant.